1. Field of the Invention
The present invention relates to a photocathode which is applicable to an image intensifier or a photomultiplier tube.
2. Related Background Art
Conventional photocathodes employing GaN are disclosed in Japanese Patent Application Laid-Open No. S61-267374 (U.S. Pat. No. 4,618,248), Japanese Patent Application Laid-Open No. H8-96705, U.S. Pat. No. 5,557,167 and U.S. Pat. No. 3,986,065. Such photocathodes have a sapphire substrate and a superlattice structure of AlGaN formed on the sapphire substrate.
The detection sensitivity of an electron tube employing a photocathode having a Group III-V nitride semiconductor layer (semiconductor layer of a nitride containing one or more elements selected from groups III-V of the periodic table), such as a GaN semiconductor layer, formed on a sapphire substrate depends on the crystallinity and surface cleanliness of the Group III-V nitride semiconductor layer. For improving characteristics of the Group III-V nitride semiconductor layer, heat treatment such as annealing and thermal cleaning is effective. Because a sapphire substrate has a relatively high transmissivity for ultraviolet rays, a photocathode employing the sapphire substrate can detect ultraviolet rays with a high efficiency. However, for not having a high absorbance for infrared rays, a sapphire substrate is difficult to be heated at a high speed in manufacturing the photocathode. Therefore, improvements in characteristics of the Group III-V nitride semiconductor layer by rapid heat treatment cannot be expected. The present invention has been made in view of these problems and is aimed at the provision of a photocathode which has improved characteristics and with which the throughput in manufacturing the same can also be improved.
With a view toward solving the above problems, a photocathode of the present invention comprises a UV glass substrate having one surface adapted to receive incident UV rays, an alkali-metal containing layer containing an alkali metal, and a Group III-V nitride semiconductor layer interposed between the other surface of the UV glass substrate and the alkali-metal containing layer and adapted to release electrons in response to incidence of the ultraviolet ray. The ultraviolet rays which have passed through the UV glass substrate are introduced into the Group III-V nitride semiconductor layer, where electrons are produced. The produced electrons are introduced into the alkali-metal containing layer containing an alkali metal such as Csxe2x80x94O and can be emitted into a vacuum therethrough. A UV glass has higher absorbance for infrared rays and higher transmissivity for ultraviolet rays than sapphire. Thus, when the UV glass is employed as a substrate, the detection sensitivity for ultraviolet rays can be improved and both the substrate and the Group III-V nitride semiconductor layer provided on the substrate can be heated at a high speed.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.